1. Field of the Invention
The present invention relates to a process and an apparatus for producing a diamond film on a substrate from a gas phase.
2. Description of the Related Art
Diamond is one of the hardest materials known, having a Vickers hardness number of 10000, and has a high Young's modulus, a good wear resistance, and a high chemical stability, and further, having a high thermal conductivity of 2000 W/mK, five times that of copper, a good insulation and a low permitivity, and remaining transparent in a wide wavelength range of light from the infrared through to the ultraviolet, and becoming semiconductive when doped with an impurity.
Because diamond has such excellent properties, it is not only essential as a tool material in high-technology fields but also is used for a variety of applications, including wear resistant coatings, vibrating elements of loudspeakers, transparent coatings of optical instruments, passivation films of semiconductor elements, heat sinks, circuit boards, anti-environmental transistors, and blue light emitting diodes, etc.
Recent studies have led to the proposals of various processes for producing diamond from a gas phase, to be thus able to better utilize these superior properties of diamond.
Hirose et al., "New Diamond", 1988, No. 10, page 34, proposed a process for a fast production of diamond on a substrate placed in the inner flame of an oxygen-acetylene flame. As generally shown in FIG. 1, acetylene gas 302 and oxygen gas 303 are introduced into a torch 301 through the bottom thereof, discharged through the torch top separately or after mixed within the torch 301, and then burnt to form a flame composed of inner and outer flames 304 and 305. The inner flame 304 is applied to a substrate 307 fixed to the lower side surface of a substrate holder 308 cooled with a coolant water 309, to precipitate diamond 306 on the surface of the substrate 307 held in the inner flame 304.
This process is advantageous in that good quality diamond is quickly produced by an extremely simple apparatus and without the use of electric power, but has a disadvantage in that the source gas is limited to an oxygen-acetylene gas, to obtain the high temperature required for the growth of diamond. Acetylene is more dangerous than other hydrocarbon gases, and therefore, is not available in the form of a high pressure bomb, and thus the use thereof requires a frequent replacement of the bombs and is very expensive.
Moreover, this process is essentially disadvantageous in that it generally provides diamond in the form of a granule, and it is difficult to thereby obtain a diamond film useful for a variety of applications.
Another problem with the conventional process is that the adhesion of a diamond film produced from a gas phase is too poor to allow it to be utilized in the above variety of applications.
To improve the adhesion of the diamond film, processes have been proposed by which the diamond nucleation density is increased or an intermediate layer of carbides or other substances is produced, but these proposals are not successful.
The present inventors proposed a DC plasma jet CVD process, which is a high speed process for producing diamond from a gas phase (Japanese Unexamined Patent Publication (Kokai) No. 64-33096), and proposed an improved process in which a metal or ceramics powder is fed to a plasma during the diamond growth by the DC plasma jet CVD process, to form a thermal-sprayed layer containing diamond as an intermediate layer inserted between an innermost metal or ceramics thermal-sprayed layer and an outermost diamond film, to thereby improve the adhesion of the diamond film (Japanese Unexamined Patent Publication (Kokai) No. 2-22741).
This process is carried out by an apparatus, as shown in FIG. 2, comprising a cathode 102 connected to the negative port of a DC power supply 104, an anode 101 connected to the positive port of the DC power supply 104 and surrounding the cathode 102, and a flow path for supplying a diamond forming gas 103 used for the diamond growth provided between the cathode 102 and the anode 101. This apparatus also comprises a powder supply nozzle 105 provided around the anode 101 and another flow path, for a carrier gas 106 conveying a metal or ceramics powder, provided between the anode 101 and the powder supply nozzle 105.
In a process carried out by using this apparatus, a diamond forming gas 103 and a carrier gas 106 containing a powder are mixed at the outlet of the powder supply nozzle 105, to form a plasma jet 110 which is applied to a substrate 107.
In the initial stage of the application of the plasma jet, the powder-containing carrier gas 106 is supplied at a rate or proportion greater than that of the diamond forming gas 103, to thus form a thermal-sprayed layer 108, and then the proportion thereof is gradually reduced or the proportion of the diamond forming gas 103 is increased, to finally form a diamond film 109.
This process has an advantage of forming an intermediate layer consisting of a thermal spray material and diamond between a thermal sprayed layer 108 and a diamond film 109, to thereby continuously produce a highly adhesive diamond film at a high growth rate.
The process, however, has a disadvantage in that, because a DC are discharge is used, the electrode materials of the anode 101 and the cathode 102 are included in the product diamond film 109.